1. Field of the Invention
The invention is related to the field of disk drive systems, and in particular to disk drive systems and circuitry that detect Gray codes in servo fields in the presence of phase shift.
2. Statement of the Problem
A disk drive system stores user data in tracks on its disks. The user data in the tracks is interspersed with servo data. The servo data includes Gray codes that identify the various tracks by cylinder number. The Gray codes may also identify head numbers and auxiliary data. The disk drive system uses the cylinder number to locate a target track before it can transfer selected user data. Because Gray codes are used to locate the user data tracks, Gray code detection is critical to proper system operation.
Gray Codes are encoded at regular intervals in the tracks and include a bit sequence that is used to detect the Gray codes. To detect a Gray code, a head is positioned over one of the tracks and senses the data encoded on the disk to generate a read signal. The disk drive system converts the read signal into a sequence of samples. The samples are passed through a matched filter to detect the Gray code detection bit sequence.
The matched filter typically operates well when the Gray codes are aligned from one track to the next. Unfortunately, the matched filter may fail during a transition between tracks if the Gray codes in the respective tracks are not aligned. The mis-alignment causes a phase shift that the matched filter cannot effectively handle. The failure to detect Gray codes interrupts system operation and inhibits the transfer of user data.
The invention solves the above problem by using circuitry that detects Gray codes in the presence of phase shift. The Gray code detection circuitry accurately detects mis-aligned Gray codes when the system transitions from one track to another. System operation is improved because Gray code detection failure is reduced resulting in fewer interruptions during user data transfer.
The invention includes disk drive systems, circuitry, and methods of operation. The disk drive system includes a disk device coupled to control circuitry. The control circuitry includes a read channel with a detector that detects a bit sequence associated with Gray codes and delays indications of the bit sequence detection to handle phase shifts. The detector generates a Gray code detection signal in response to the delayed indications. The control circuitry processes the Gray code in response to the Gray code detection signal.